A shutter mechanism allows light to pass for a determined period of time. As such, shutters can be used to expose photographic film or a light-sensitive electronic sensor to a preferred amount of light to create an image of a view. A shutter can also be used to allow pulses of light to pass outwards, as in a movie projector or signal lamp. Thus, the shutter is positioned to selectively interfere with a beam of light. In an optical system, the shutter can be mounted behind a lens (central shutters), or near the focal plane (focal plane shutters). The focal plane shutter is a fairly simple mechanism, capable of rapid and accurate shutter speeds.
FIG. 1 depicts a front view of a conventional focal plane shutter 10 for selectively covering and exposing a frame aperture 16 through which a large format exposure is made. Focal plane shutters 10 are usually implemented as a pair of cloth, metal, or plastic curtains that shield film from light. For example, the shutter 10 includes two screens 12, 14 positioned in a plane parallel to the frame aperture 16. The screens 12, 14 are slideable between open and closed positions. In the closed position, one or more of the screens 12, 14 are positioned to substantially cover the frame aperture 16. In the open position, one ore more screens 12, 14 are repositioned to leave at least a portion of the frame aperture 16 uncovered. Exposure times vary from several seconds to a small fraction of a second, depending upon the particular application. In order to support short exposures, the screens 12, 14 can be rapidly and or simultaneously moved between the open and closed positions. One example of such a shutter is a “Bonn shutter” described below.
Large-format exposures are associated with photographic films, view cameras, and processes that use a film or digital sensor having dimensions of 4×5 inches or more. The most common formats are 4×5 and 8×10 inches. Large format exposures are often used in landscape photography, advertising photos of high value consumer items, fine-art photography, images that will be enlarged to a high magnification, and scientific applications demanding very high quality reproductions. One particular application includes producing high-quality astronomical images using optical telescopes. Current techniques capture an image using an electronic detector, such as a charge-coupled device (CCD).
In some applications, the screens 12, 14 used for large format exposures are provided as curtains that roll on and off of respective spools located to the side of the frame aperture 16. A curtain screen may be ineffective in providing a precise exposure due to sagging of the curtain across the large aperture. Others have addressed this problem using rigid screens such as those used with the Bonn shutter developed at the Astronomical Institutes of the University of Bonn. The Bonn shutter, developed for a scalable large format exposure shutter for CCD mosaic cameras, includes a slit-type shutter with a rectangular aperture. The shutter includes two carbon fiber blades moving on a pair of linear ball bearings.
Although the Bonn shutter provides at least one solution to large format exposure shutters, there are limitations. First, the rigid blades are relatively massive thereby complicating rapid accelerations due to their inertia. Second, the rigid blades require substantial clearance along either side of the aperture 16 to accommodate the blade in an open position. As shown in FIG. 1, the minimum width required for a Bonn shutter is WS1, which at a minimum includes the width of the aperture WA combined with the widths of each of the blades 12, 14.